1. Field of the Invention
The invention relates to a cermet for a lamp which is used for hermetically sealing a discharge vessel in a ceramic discharge lamp, and also to a ceramic discharge lamp. The invention relates especially to a cermet for a lamp with a coefficient of linear expansion which is essentially equal to the coefficient of linear expansion of a translucent ceramic which forms the discharge vessel, and to a discharge vessel of ceramic with hermetically sealed components such a cermet for a lamp.
2. Description of the Related Art
Conventionally a ceramic discharge lamp which is shown in FIG. 1 (for example patent publication JP-SHO 61-220265) is known which has a discharge vessel 3 of translucent ceramic, with an arc tube part 1 and side tube parts 2 which are joined to the arc tube part 1. In the arc tube part 1, there are a pair of electrodes 4 opposite one another. The electrodes 4 are located in 5 the tip areas of the upholding parts 5 of the electrodes. The base parts of the upholding parts of the electrodes are inserted into hermetically sealed components 6. A hermetically sealed arrangement is obtained by fritting-welding of these hermetically sealed components 6 in the side tube parts 2. In FIG. 1, reference number 7 labels an outer lead which is inserted into the hermetically sealed component 6.
The hermetically sealed component 6 in this discharge lamp consists of a conductive cermet which is obtained by sintering of ceramic powder and metal powder and is hermetically welded by a glass frit (not shown in the drawing) in the side tube part 2.
The ceramic for obtaining this cermet is the same material as the translucent ceramic comprising the discharge vessel 3, for example polycrystalline aluminum oxide, so that the difference between the coefficient of linear expansion of the hermetically sealed component 6 of this cermet and the coefficient of linear expansion of the discharge vessel 3 is reduced.
Molybdenum or tungsten is used as the metal powder to obtain the cermet. To ensure the conductivity necessary for supply, a metal with a percentage by volume from 30 to 60% is contained therein.
Metals such as molybdenum and tungsten which are contained in the cermet have a smaller coefficient of linear expansion than a ceramic like aluminum oxide. The coefficient of linear expansion of the cermet which comprises the hermetically sealed component 6 is therefore, less than the coefficient of linear expansion of the ceramic which comprises the cermet. As a result, between the coefficient of linear expansion of the cermet to be obtained and the coefficient of linear expansion of the translucent ceramic comprising the discharge vessel 3, a difference of greater than 1xc3x9710xe2x88x926 (Kxe2x88x921) results, even when the same material as that of the translucent ceramic comprising the discharge vessel 3 is used as the ceramic for the cermet.
As a result, the following disadvantages arose when the hermetically sealed components are formed for example from a conventionally known conductive cermet (especially based on aluminum oxide-molybdenum) and from fritting-welding these hermetically sealed components on the discharge vessel of translucent aluminum oxide ceramic.
In welding (cooling process) or at the start of operation (within a few hundred hours), there are cases in which due to the different coefficients of linear expansion of the two material components, cracks form at the welded sites. Reliability is therefore low.
The first object of the invention is to devise a cermet for a lamp in which the reliability of the hermetically sealed parts is increased by the fact that their coefficient of linear expansion is made essentially equal to the coefficient of linear expansion of a translucent ceramic which comprises the discharge vessel of a discharge lamp of ceramic.
A second object of the invention is to devise a cermet for a lamp in which no cracks form at the weld spots, even if it is frit-welded as the hermetically sealed components of a discharge lamp of ceramic in the side tube parts of the discharge vessel.
The third object of the invention is to devise a ceramic discharge lamp in which no cracks occur at the locations at which side tube parts of the discharge vessel and the hermetically sealed components are frit-welded to one another.
The objects are achieved by a cermet in accordance with the embodiments of the present invention where.
(1) In a cermet for a lamp which is used for hermetically sealing a discharge vessel (10) in a ceramic discharge lamp, the object is achieved in that the cermet contains a material with a coefficient of linear expansion which is greater than or equal to that of a translucent ceramic comprising the discharge vessel (10) of the above described ceramic discharge lamp. Furthermore, the cermet contains a material with a coefficient of linear expansion which is smaller than that of the translucent ceramic, and that the average coefficient of linear expansion at 25 to 300xc2x0 C. is in the range of Exc2x11.0xc3x9710xe2x88x926 (Kxe2x88x921) where E (Kxe2x88x921) the average coefficient of linear expansion of the translucent ceramic comprising the discharge vessel (10) of the above described ceramic discharge lamp at 25 to 300xc2x0 C.
(2) In a cermet for a lamp which is used for hermetically sealing a discharge vessel (10) in a ceramic discharge lamp, the cermet is obtained by sintering with one another a ceramic with a coefficient of linear expansion greater than that of a translucent ceramic comprising the discharge vessel (10) of the above described ceramic discharge vessel, and a metal with a coefficient of linear expansion less than that of the translucent ceramic.
(3) In a cermet for a lamp which is used for hermetically sealing a discharge vessel (10) in a ceramic discharge lamp, the object is achieved in that the cermet is obtained by sintering with one another a ceramic, a metal with a coefficient of linear expansion which is less than that of a translucent ceramic comprising the discharge vessel (10) of the above described ceramic discharge lamp, and a material with a coefficient of linear expansion greater than that of the translucent ceramic.
In this cermet for a lamp it is desirable for the average coefficient of linear expansion of the ceramic which forms the cermet at 25 to 300xc2x0 C. to be in the range of Exc2x11.0xc3x9710xe2x88x926 (Kxe2x88x921) where E (Kxe2x88x921) the average coefficient of linear expansion of the translucent ceramic comprising the discharge vessel (10) of the above described ceramic discharge lamp at 25 to 300xc2x0 C. It is especially preferred that the ceramic which forms the cermet comprises of the same material as the translucent ceramic comprising the discharge vessel (10) of the above described ceramic discharge lamp.
Furthermore the objects in the ceramic discharge lamp in accordance with the embodiments of the present invention achieved as follows:
(4) In a ceramic discharge lamp including a discharge vessel (10) of translucent ceramic with an arc tube part (11) and side tube parts (12) which are connected to the arc tube part (11), further including a pair of electrodes (21) opposite one another in the arc tube part (11) and in which a hermetically sealed arrangement is obtained by fritting-welding of these hermetically sealed components (24) on the side tube parts (12). The base parts of the upholding parts (22) of the electrodes are inserted, into the hermetically sealed arrangement the upholding parts having trip areas which are provided with electrodes (21) the object is achieved as claimed in the invention in that the above described hermetically sealed components (24) consist of the above described cermet for a lamp.
(5) In a ceramic discharge lamp including a discharge vessel (10) of translucent ceramic with an arc tube part (11) and side tube parts (12) that are connected to the arc tube part (11), further including a pair of electrodes (21) opposite one another in the arc tube part (11) and in which a hermetically sealed arrangement is obtained by fitting-welding of cylindrical or disk-shaped hermetically sealed components (24) on the outside faces of the side tube parts (12). The base parts of the upholding parts (22) of the electrodes are inserted into the hermetically sealed arrangement the upholding parts having tip areas which are provided with electrodes (21) the object is achieved as claimed in the invention in that where the above described hermetically sealed components (24) consist of the above described cermet for a lamp.